Liquid crystalline polyesters (LCP's) are unique among polymers because they have very high tensile, flexural, and temperature resistance properties which are very desirable for high performance applications, such as in structural applications and in electrical and electronic applications. Another property that is highly desirable for the electrical and electronic applications is a high resistance to burning. This property is most often measured by the Underwriters Laboratories Flammability Test (UL-94). A rating of V-O (which is the best rating possible from the test) is desired for the electrical and electronic applications.
Among the earliest LCP's available were those derived from poly(ethylene terephthalate) and p-acetoxybenzoic acid. These LCP's had the disadvantages of relatively poor temperature resistance and inadequate flammability characteristics. Subsequently, all-aromatic LCP's then were developed and commercialized. Typical of these all-aromatic LCP's are those described in U.S. Pat. No. 4,169,933 and derived from terephthalic acid, 2,6-naphthalenedicarboxylic acid, hydroquinone, and p-hydroxybenzoic acid. Relatively thin, e.g., less than 1.5 mm, articles having good tensile and flexural modulus properties, and high thermal (heat distortion temperature) properties are employed as circuit boards in the electrical/electronic industry. For such electronic applications, it is important that these thin articles exhibit excellent resistance to burning, e.g., a classification of 94V-Oas determined by the UL-94 20 mm Vertical Burning Test (ASTM D 3801).
U.S. Pat. No. 5,124,397 discloses a resin composition for sliding movement comprising (a) certain thermotropic, liquid crystalline polyesters and (b) 10 to 70 weight percent spherical glassy carbon. Most of the disclosed compositions also contain graphite, glass fiber or poly(tetrafluoroethylene). The flammability characteristics of the resin compositions are not mentioned in the patent. Japanese Published Patent Application JP 08-220,407 A discloses a light pickup lens holder comprising a filled, flame-retardant liquid crystal resin. PCT International Application WO 96/06888 discloses LCP's which contain a small amount of low molecular weight polyethylene. The polyethylene acts as an internal lubricant or mold release but does not significantly increase the flammability of the LCP composition. Example 4C of this PCT patent document discloses an LCP composition consisting of (i) 61.5% of an LCP containing residues of hydroquinone (9.6%), 4,4'biphenol (9.6%), terephthalic acid (13.5%), 2,6-naphthalenedicarboxylic acid (5.8%), and p-hydroxybenzoic acid (61.5%). (ii) 0.5% oxidized low pressure polyethylene, (iii) 23.3% fiberglass, (iv) 1.3% carbon black and (v) 13.3% talc. The LCP compositions contemplated by this PCT patent document are intended for use in the manufacture of main parts in driving devices of copying machines, printers and video-related equipment which require strong mechanical and high tribology characteristics. Flammability characteristics of the LCP compositions are not mentioned.
Japanese Published Patent Application JP 08-13,248 A discloses fire-resistant black polyester fibers prepared by mixing and melt spinning a master batch containing 5-40 weight percent carbon black and 60-95 weight percent poly(ethylene terephthalate)-based copolymer polyester prepared by copolymerization of poly(ethylene terephthalate) with 5-20 mole percent of an isophthalic acid component, and a matrix polymer consisting of poly(ethylene terephthalate) containing no carbon black. Liquid crystalline polyesters are not contemplated by this reference. Japanese Published Patent Application JP 07-196,894 A discloses a liquid crystalline resin composition consisting of 0.01-10 weight percent carbon black and a liquid crystalline polyesteramide or polyester forming an anisotropic melt phase. The liquid crystalline resin compositions also may contain 0.5-60 weight parts of a flame retardant organobromine compound per 100 parts of the liquid crystalline resin. JP 07-196,894 A discloses flammability characteristics only for resin compositions which contain a flame retardant amount of an organo-bromine compound which suggests that the presence of such a flame retardant is essential to an acceptable flammability rating.